Verticillium species belong to the phylum Ascomycota, which comprises the largest group of fungal (plant) pathogens. The genus Verticillium contains three major plant pathogenic species: V. dahliae, V. albo-atrum, and V. longisporum (Fradin and Thomma, 2006; Klosterman et al., 2009). While V. dahliae and V. albo-atrum infect over 200 plant species, V. longisporum is pathogenic mainly on Brassicaceae. The pathogens cause soil-borne vascular wilt, which is a devastating disease on many economically important crop species such as tomato, potato, cotton, and lettuce, but also on ornamental plants (Agrios, 2005; Fradin and Thomma, 2006; Klosterman et al., 2009). Controlling Verticillium wilt disease is difficult for several reasons: Verticillium produces resting structures that can survive in the soil for many years (Rowe and Powelson, 2002), it has a broad host range, and the fungus is difficult to be reached by fungicides once it has entered the xylem tissue. A commonly used control option, crop rotation, is mostly ineffective for controlling Verticillium wilt disease. Although soil fumigation is effective to control Verticillium wilt disease, use of soil fumigation is not appreciated due to the detrimental effects of the chemicals on public health and the environment. Soil fumigation is also not a preferred method for large scale field application. As a consequence, the preferred method to control Verticillium wilt disease is the use of genetic resistance.
Two distinct races (race 1 and race 2) have been described for V. dahliae and V. albo-atrum in tomato and lettuce (Fradin and Thomma, 2006; Klosterman et al., 2009). While resistance against race 1 strains has been identified in these two plant species (Schaible, 1951; Fradin et al., 2009; Hayes et al., 2011), no resistance against race 2 has been identified so far. Genetic resistance against Verticillium wilt diseases has also been reported for several other economically important crop species (Pegg, 2002). However, so far the only Verticillium resistance locus that has been cloned and functionally characterized is the tomato Ve locus that contains the Ve1 gene that provides resistance in tomato against race 1 isolates of V. dahliae and V. albo-atrum (Kawchuk et al., 2001; Fradin et al., 2009).
Recently, it has been shown that transgenic expression of Ve1 in Arabidopsis provides resistance against Verticillium race 1 isolates (Fradin et al., 2011). Over the years, Arabidopsis has increasingly been used as a model host for studying Verticillium-host interactions (Veronese et al., 2003; Tjamos et al., 2005; Fradin and Thomma, 2006; Johansson et al., 2006; Ellendorff et al., 2009; Pantelides et al., 2010b). In addition to screening germplasm for resistance (Schaible, 1951; Veronese et al., 2003), mutagenesis followed by screening for enhanced resistance with a pathogen of interest is a means to identify novel resistance traits. Several molecular techniques have been used to generate random mutants in Arabidopsis, such as EMS- and radiation-induced mutation, and transposon and activation tagging. Activation tagging involves the random integration of promoter or enhancer sequences in a genome, using either a T-DNA or a transposon, generally leading to enhanced expression of genes near the integration site and generating gain-of-function mutations (Weigel et al., 2000; Ayliffe and Pryor, 2007; Pereira and Marsch-Martinez, 2011). Insertion of enhancer sequences in the genome may positively regulate gene expression, even when inserted at a considerable distance to the target gene (Lewin, 2008). Some of the advantages of activation tagging over knock-out strategies include that activation tagging generates dominant instead of recessive mutations, it generates viable mutants for those genes where knock-outs would lead to lethal phenotypes and it is also applicable to dissect phenotypes of redundant genes (Pereira and Marsch-Martinez, 2011).